1. Field of the Invention
The present invention relates to an aluminium nitride sintered body, and more specifically to an aluminium nitride sintered body having a conductive metallized layer with an excellent high-temperature adhesion to the aluminium nitride sintered body base material and available for a semiconductor device substrate.
Furthermore, the present invention relates to a circuit substrate using an aluminium nitride sintered body as an insulating layer, and more particularly to a circuit substrate in which conductive layers are integrally and simultaneously sintered with the insulating layer to realize a multilayer interconnection.
2. Description of the Prior Art
Owing to excellent high insulation characteristics, corrosion resistance, thermal shock, high-temperature strength, thermal radiation, and heat conductivity, aluminium nitride (AlN) sintered bodies have been noticed as various structure materials, various electronic/electric parts materials, etc. Recently, in particular, this AlN sintered body has been used in place of alumina (Al.sub.2 O.sub.3) and beryllia (BeO), because Al.sub.2 O.sub.3 is not satisfactory in heat radiation and BeO is poisonous and therefore complicated to handle. When the AlN sintered body is used, the body is usually joined with some metal members, so that a conductive metallized layer is generally formed on the surface of the AlN sintered body.
As a method of forming a metallized layer on the AlN sintered body, there is well known the direct bond copper (DBC) method by which an oxide layer (Al.sub.2 O.sub.3) is formed on the surface of the sintered body and thereafter copper (Cu) foil is directly bonded or the thick film method of Copper (Cu), gold (Au), silver (Ag)-palladium (Pd).
However, the conductive metallized layer formed on the surface of the AlN sintered body in accordance with the above-mentioned conventional methods is weak in terms of adhesion to the AlN sintered body, in particular at high temperature. Therefore, there exist problems such that it is difficult to bond another member with the formed metallized layer by a high temperature (700.degree. C. or higher) bonding method such as brazing, high temperature soldering, etc. or, even if bonded, when the AlN sintered body to which another member is bonded is used at high temperature, the formed metallized layer peels off the surface of the sintered body and eventually another member is dropped off from the metallized layer. In particular, where the AlN sintered body including a conductive metallized layer is used as a heat radiation substrate for electronic circuits, since the substrate is subjected to temperature changes (heat cycles) from low to high or vice versa, there exists another disadvantage such that cracks develop in the metallized layer due to a difference in thermal expansion coefficient between the AlN and the metallized layer forming component.
This AlN sintered body is manufactured roughly as follows:
An AlN powder is first mixed with a sintering aid such as Y.sub.2 O.sub.3, Sm.sub.2 O.sub.3, CaO, etc. of a predetermined amount, and further with an acrylic base resin binder, if necessary as an aid. These are sufficiently mixed, formed into an AlN green sheet body (raw compact) of a predetermined shape under pressure, and sintered at a predetermined temperature within a nitride atmosphere, for instance.
Where the AlN sintered body is used as a substrate for semiconductor devices, it is necessary to further form a conductive film on the surface of this AlN sintered body. Conventionally, this film was a metallized layer of Cu, Au or Ag-Pd formed on the surface of the AlN sintered body in accordance with the DBC (direct bond copper) method or the thick film method.
However, these conventional substrates involve the following problems:
The first problem is that the adhesion strength between the metallized layer and the AlN sintered body surface is weak, and therefore peeling occurs between the two as shown in FIG. 1A, thus lowering the substrate reliability, as shown in FIG. 1A.
The second problem occurs when a semiconductor element or wire is brazed or soldered at a high temperature onto the formed metallized layer. That is to say, although brazing is effected at about 800.degree. C. with a hydrogen-nitrogen mixture gas, since the metallized layer baking temperature is as low as about 600.degree. to 1000.degree. C., the adhesion junction strength between the metallized layer and the AlN sintered body surface is lowered markedly in brazing, thus disabling brazing in practice. Further, a similar problem arises in the case of high temperature soldering.
The third problem occurs due to a difference in thermal expansion coefficient between the AlN sintered body and the metallized layer. As is the case of brazing and high temperature soldering, severe heat-cool cycles are applied in use to the substrate on which semiconductor elements such as silicon wafers are mounted. As a result, thermal stresses are generated on junction surfaces between AlN sintered body, metallized layer, brazed layer (or soldered layer), and semiconductor elements due to differences in thermal expansion coefficient between two of these layers, thus resulting in an occurrence of peelings of these elements, as shown in FIG. 1B.
The thermal expansion coefficient of the metallized layer is about 2 to 4 times greater than that (about 4.6.times.10.sup.-6 /.degree.C.) of the AlN sintered body, and roughly equal to or half of that of the brazed or soldered layer. Therefore, since there is a big difference between the AlN sintered body and the metallized layer or the brazed (soldered) layer, microcracks are readily produced on the boundary surface between the AlN sintered body and the metallized layer or the brazed (or soldered) layer during heat cycle. Further, these microcracks develop gradually when the heat cycle is repeated, thus finally causing the peeling-off of the semiconductor element from the sintered body. This problem is serious, in particular, because the reliability of appliances having substrates on which semiconductor elements are mounted will be deteriorated.
The fourth problem is that the adhesion strength between the metallized layer and the AlN sintered body is small at high temperatures, so that the reliability of the substrate is low at high temperature as in the second problem.
Finally, the fifth problem is that thermal energy is not economical when checking the manufacturing processes from the first to the last. In other words, the substrate is completed by first sintering an AlN green body and further sintering the AlN sintered body again to form the metallized layer.
With the advance of miniaturization of electronic appliances, the way heat generated from semiconductor devices mounted on a circuit substrate can effectively be radiated has become an important problem. Further, heat radiation is an important problem when power semiconductor devices are mounted on a circuit substrate.
Conventionally, Al.sub. O.sub.3 ceramics have widely been adopted as an insulating material for circuit substrates. However, since the Al.sub.2 O.sub.3 is low in heat conductivity, there still exists a problem with heat radiation. Therefore, application of AlN ceramics excellent in electrical characteristics such as electric insulating characteristics (as an insulator) and in thermal conductivity to circuit substrates has been studied (as disclosed Japanese Published Unexamined Pat. Appl. No. 60-178688).
When taking into account the miniaturization and high densification in electronic appliances, a higher densification is also required for wiring on circuit substrates and therefore a multilayer AlN ceramic substrate has been studied (as disclosed Japanese Published Unexamined Pat. Appl. Nos. 60-253294 and 60-253295). Further, in the Al.sub.2 O.sub.3 ceramics, although a technique of simultaneously sintering a plurality of laminated green sheets has been established, it is impossible to simply apply the technique for Al.sub.2 O.sub.3 ceramics to AlN ceramics as it is, because of a difference in fundamental properties between AlN and Al.sub.2 O.sub.3.
As described above, although the demand for AlN ceramic based multilayer interconnection circuit substrates has increased, since warp, conductive path disconnection, peeling-off, etc. will be produced during the process of simultaneous sintering, the AlN multilayer ceramics is not practical at present.
In summary, with the advance of higher speed, higher densification, and higher output power of semiconductor devices mounted on a substrate, there exists a strong demand for AlN sintered bodies provided with higher heat conductivity, higher adhesion strength, excellent electric characteristics and additionally with multilayer interconnection owing to simultaneous sintering of the AlN bodies and metallized layers.